1. Field of the Invention
The present invention relates to a colored ceramic composition and a method for producing a curved glass plate employing it.
2. Discussion of Background
Heretofore, a colored ceramic paste for an automobile has been widely used which is designed so that a colored ceramic composition in the form a paste is screen-printed along the peripheral portion or at the center portion of a window glass of an automobile, then dried and baked in a step of heating the glass plate for bending.
When baked along the peripheral portion of the window glass, this colored ceramic paste will form a colored opaque layer which is useful for the purpose of preventing deterioration of a urethane sealant by ultraviolet rays or preventing e.g. terminals of heating wires from being seen from outside the automobile. Known as compositions of this type are those having various heat-resistant coloring pigments mixed to low melting point amorphous glass or crystallized type glass which usually contains lead or cadmium, as a base material.
On the other hand, as a glass composition which does not contain lead and cadmium, a zinc-containing system having the following composition has been known (Japanese Examined Patent Publication No. 43296/1986):
______________________________________ SiO.sub.2 10 to 40 wt % B.sub.2 O.sub.3 10 to 35 wt % ZnO 3 to 51 wt % Al.sub.2 O.sub.3 1 to 15 wt % Li.sub.2 O + Na.sub.2 O + K.sub.2 O 1 to 20 wt % TiO.sub.2 + SnO.sub.2 0 to 5 wt % CaO + MgO + BaO 0.5 to 35 wt % F 0 to 2 wt % ______________________________________
Further, U.S. Pat. No. 5,342,810 discloses a glass frit having the following composition:
______________________________________ ZnO 31 to 50 mol % SiO.sub.2 10 to 44 mol % B.sub.2 O.sub.3 11 to 35 mol % Na.sub.2 O 11 to 25 mol % Li.sub.2 O 0 to 4 mol % TiO.sub.2 0 to 10 mol % ZrO.sub.2 0 to 25 mol % CaO 0 to 12 mol % SrO 0 to 12 mol % BaO 0 to 12 mol % F 0 to 8 mol % ______________________________________
However, the compositions specifically disclosed in Examples of these two documents have a relatively small content of SiO.sub.2 and a large content of Na.sub.2 O, and accordingly, they are believed to be inferior in chemical durability, particularly acid resistance.
Further, U.S. Pat. No. 5,306,674 discloses a glass frit having the following composition:
______________________________________ ZnO 20 to 40 wt % B.sub.2 O.sub.3 20 to 32 wt % SiO.sub.2 10 to 30 wt % TiO.sub.2 0 to 12 wt % Na.sub.2 O 4 to 12 wt % K.sub.2 O 0 to 10 wt % ZrO.sub.2 0 to 12 wt % Al.sub.2 O.sub.3 0 to 4 wt % Li.sub.2 O 0 to 5 wt % BaO 0 to 10 wt % CaO 0 to 5 wt % SrO 0 to 5 wt % MgO 0 to 5 wt % F 0 to 5 wt % ______________________________________
As a bending method for a window glass for an automobile, a production mode of a system so-called an in-furnace bending method has been employed in recent years for the purposes of improving the productivity and precision in bending, wherein a pressing apparatus or a bend adsorption-shaping apparatus is provided in a heating furnace, and bending is carried out in the furnace (at a temperature of from 570 to 700.degree. C.). If the above-mentioned colored ceramic paste is used for the in-furnace bending method, the colored ceramic composition is likely to adhere to the heat-resistant cloth such as a glass cloth which is usually employed on the surface of the pressing die or the adsorption die for bending, whereby the so-called release property tends to be poor, and the productivity tends to decrease, such being undesirable.